One possibility is a large beetle from the scarab family. I found a kind of Rhinoceros beetle in my son’s sandpit recently, in a tunnel about the size mentioned. A ways down, had to dig for it…

They are not uncommon especially if you use mulch.

These particular beetles (Xylotrupes species) ‘squeak’ quite loudly when they feel threatened, which might explain your dog’s interest Mia80. All bluff, they are totally harmless (the sound is actually their wing covers being rubbed on their abdomen).

Have a close look at the hole/tunnel. If there are no signs of threads, it is unlikely to be a spider.